Ecological dynamics of Anticosti island kelp forest: insights into subarctic rocky reef ecosystems from eastern CanadaReef-associated Biodiversity

Fanny Noisette ¹, Christian Nozais¹, Simon Bélanger¹, Ladd Johnson², Stephanie Roy¹, Raphaël Mabit¹

¹Université du Québec, Canada - ²Université Laval, Canada

Kelp forests are rich and productive ecosystems found in every temperate to polar ecosystems, but not well mapped and described in the Saint-Lawrence system (marine estuary and Gulf). In the middle of the Gulf, the broad rocky flats and cold-transparent waters of Anticosti Island are home to kelp forests that are likely the vastest in the Saint-Lawrence system, but still largely unknown. From 2021 to 2024, scuba-diving surveys were carried out to characterize kelp communities in a dozen of sites around the southwest region of Anticosti Island. Five kelp species (Saccharina latissima, Alaria esculenta, Hedophyllum nigripes/Laminaria digitata, Agarum clathratum, Saccorhiza dermatodea) were observed from the surface to 15m deep, with great variability in assemblages between sites and from one year to another. Densities of 10 to 100 individuals m-2, and biomasses of 0.5 to 7 kg m-2, were similar to other kelp forests in eastern Canada. Pluriannual dynamics of kelp communities could not be related to grazing pressure by urchins though. Community composition was dominated by Saccharina latissima and Alaria esculenta at shallow depths (up to 7m), being replaced Agarum clathratum deeper, in spite of light conditions allowing kelp growth up to 30 m. This description of the kelp forests of southwestern Anticosti Island was completed by water column chemical, optical and physical characterization, opening up new perspectives on the ecological processes driving the dynamics of these understudied coastal zones.